Dual-band antennas are well-known and are widely used in a variety of wireless communication devices. In particular, dual-band antennas have become popular in connection with portable cellular devices such as cellular radiotelephones because they allow these devices to operate within more than one wireless communication system. For instance, a cellular phone having dual-band capability allows a user to access one or more cellular systems that may be present in a given region, or more importantly, the ability to select service from a plurality of systems so that the user can access at least one of the systems where, for example, none of the systems provides service in all the regions that are of interest to the user.
Presently, a number of commonly used cellular systems operate in different frequency bands. For example, time division multiplex access (TDMA) systems, one of which is commonly referred to as GSM, operate in bands from 890 Megahertz (MHZ) to 960 MHZ and 1710 MHZ to 1880 MHZ and are commonly used in European cellular systems. Also, for example, the Personal Communication System (PCS), which operates in a band from 1850 MHZ to 1990 MHZ, and analog systems such as AMPS, which operates in a band from 824 MHZ to 894 MHZ, are two popular cellular systems used in North America. Accordingly, some manufacturers have developed cellular phones that can operate in two or more of these popular cellular systems.
A cellular phone that operates in two or more frequency bands requires an antenna system that is capable of receiving and transmitting signals in these distinct frequency bands. In one known approach, the cellular phone includes two separate antennas that are specifically configured to receive signals in different frequency bands. Typically, one of the antennas is a retractable linear antenna and the other is a helical antenna that is located adjacent to the retractable antenna. When the linear antenna is in the retracted position, the helical antenna is active, and when the linear antenna is in the extended position, the helical antenna is shorted or otherwise disabled so that only the extended linear antenna conveys signals.
Another known approach uses a single antenna structure that has multiple resonance modes, thereby allowing the antenna to convey signals in two or more frequency bands. Typically, these dual-band antennas generate two or more modes of resonance using either two helical coils wound on a single core, or alternatively, a single coil having a variable pitch. In particular, one commercially available dual-band antenna is the model DHR-1992 from Ace Antenna Co., which is located in Chatsworth, Calif. The DHR-1992 uses a single variable pitch helically wound coil to operate at both GSM and PCS frequencies. Because of the size and packaging constraints associated with hand-held cellular phones, some manufacturers have found that a quarter wavelength "stubby" antenna with a helical winding is advantageous because it can provide a small size and a relatively broad bandwidth at two or more of the frequency ranges mentioned above.
The above-described known approaches to providing an antenna system that can operate in two or more frequency bands are relatively complex and costly. Namely, retractable antennas require a significant amount of electromechanical hardware, are physically inconvenient for a user, and may be easily damaged in the extended position. Retractable antennas are further disadvantageous because matching the antenna to transceiver circuitry in both the extended and retracted positions, while maintaining high antenna efficiency, is extremely difficult. Additionally, a stubby antenna design using two helical coils requires precise winding (e.g., precise orthogonality) of two independent coils on a single core, which can result in inconsistent performance in units that are manufactured in a factory environment. Still further, while stubby antennas having a single variable pitch helical coil are simple in construction, they require an inherent design tradeoff such that sufficient bandwidth is typically only achievable at one of the resonant frequencies.